Pretty technical, but at the bottom here's a quote:
Thiosulfate is also equally suited to dechlorinating free chlorine in water, and it has gained wide use in marine and freshwater aquaria. Unfortunately, the ammonia that is produced as a result of the reaction is still toxic. Consequently, thiosulfate alone is not always adequate for eliminating toxicity from chloramine.
Basically, the way I see it, chloramine is Nh2Cl... or one molecule of Ammonia (Nh2) bound to one molecule of Chlorine (Cl).
The Sodium Thiosulfate removes the chlorine (or neutralizes it, so you don't "kill" the fish with the chlorine), but that then leaves the ammonia (Nh2) in the tank which then ALSO affects the fish by increasing ammonia levels, which is also harmful.
So, when it says it "takes care of" chloramine, it does... in a way... the chloramine is gone, but the ammonia is still there.
It used to be that with basic chlorine treated water, all we needed to worry about was chlorine... and ST DID take care of that. Chloramine is a much more stable product in the water and does not dissipate easily as the chlorine did.
And... the more water changes you do (to then try to remove the resulting higher ammonia) just raises the ammonia/nitrites/nitrates even more.
It is better if you can find a product (like Prime, or Amquel) that ALSO neutralizes the ammonia.